Downhole tools use various types of sensors to test a downhole formation, analyze fluids, and perform other operations. Because the downhole environment has high temperatures, high pressures, harsh chemicals, and mechanical vibrations, the downhole tools must be mechanically designed to handle problems associated with such harsh conditions, and the downhole sensors must still be able to operate with analytical accuracy and reliability. Added to these challenges, the downhole sensors must fit in the limited space available in the downhole environment, must be light weight and power efficient, and have a large dynamic range. For these reasons, optical sensors are often the sensor of choice for downhole use.
In the art, spectrophotometers, spectrometers, spectrofluorometers, refractive index analyzers, and similar devices have been used to analyze downhole fluids by measuring the fluid's spectral response. Each of these device typically use some form of electromagnetic (EM) radiation to perform its function (i.e., to analyze the fluid). In general, the wavelengths of the EM radiation can be in the x-ray, gamma, ultraviolet, visible, infrared or any combination of these ranges. When the radiation is detected, the response can identify characteristics of the analyzed fluid, such as the type of fluid (e.g., oil, water, and/or gas), the level of filtrate contamination, the hydrocarbon composition (e.g., amount of methane (C1), ethane (C2), propane (C3), etc.), the gas-to-oil ratio (GOR), etc. Knowledge of these characteristics can then be employed to model the reservoir, plan production, and perform other tasks.
Typically, prior art optical devices have operational limitations in a downhole environment because of the manner in which the EM radiation must be split into various components. In addition, the number of measurement channels is limited due to the instrument design. These and other characteristics of prior devices typically result in limited capabilities for real-time, in-situ fluid analysis in a downhole wellbore environment. In any event, prior art optical devices have not incorporated dynamic real time referencing at each measurement wavelength to maintain sensor calibration, which could improve sensor performance.
For higher resolution, the optical devices are usually located at the surface to avoid the difficulties associated with the downhole environment. In these situations, fluid samples can be obtained downhole and transported to the surface for subsequent analysis. As expected, testing with this type of device does not provide prompt analysis. Alternatively, most of the electronic components of the optical device are located at the surface. Fiber optics running in the borehole carry input light from the surface component to a downhole sample. Then, fiber optics return the measurement light from the sample to the optical device's surface components so the measurement light can be analyzed. As expected, this type of device can be cumbersome and fragile and can usually only be employed in permanent installations.
Use of a filter wheel is one way to offer a number of spectral channels for analysis. For example, pharmaceutical and refining industries use filter wheel spectrometry/photometry to analyze fluids. In one example, a field photometer has a rotatable filter wheel with nine elements. See Z. Frentress, L. C. Young, and H. D. Edwards, “Field Photometer with Nine-Element Filter Wheel,” Filter Wheel Art, Appl. Opt. 3, 303-308 (1964). In another example, a process photometer disclosed in U.S. Pat. No. 7,321,428 has a filter wheel with filters. Likewise, companies such as Turner Biosystems and Sherwood Scientific offer commercial filter wheel spectrometry systems for laboratory and industrial applications. However, such laboratory-based systems are not suitable for downhole use.
What is needed is an optical device that is deployed downhole to analyze fluids and that offers a high level of spectral reproducibility and reliability with a plurality of spectral channels. Furthermore, what is needed is an optical device with improved sensor performance that uses real time referencing to account for the harsh operating conditions found downhole.